In a bowl, mix semolina, yogurt, a little salt, and some water to make a thick batter. Cover and let it rest for 10–15 minutes.
Heat a little ghee in a pan, add mustard seeds, curry leaves, and finely chopped green chilies. You can also add grated carrots or beans if desired. Add this tempering.
Now add Eno to the batter and mix immediately. The batter will start to rise. (You can also add baking soda + lemon juice instead.)
Grease the idli molds and immediately pour the batter into the molds.
Steam the idlis for 10–12 minutes on medium heat. Check with a toothpick – if it comes out clean, it's ready! Remove the idlis from the mold and serve hot with chutney.